Well, PHPUnit 8.5.8 is not (necessarily) compatible with PHP 8.
Can you reproduce the issue directly (i.e. without PHPUnit)?

Also see <https://github.com/sebastianbergmann/phpunit/issues/4373>.

I am aware that PHPUnit 8 is not compatible to PHP8, but if it is incompatible and fails for example in this case on the newly reserved keyword "match", I would expect PHP8 to throw an error that gives me the file and line number where this invalid token is encountered. However my stack trace looks like this:

42) Joomla\Tests\Unit\Libraries\Cms\Toolbar\ToolbarTest::testLoadButtonTypeRetunsFalseForUnkownButtonTypes
ParseError: syntax error, unexpected token "match", expecting identifier or "static" or "namespace" or "\"

D:\apachefriends\xampp\htdocs\j4\libraries\src\Autoload\ClassLoader.php:56
D:\apachefriends\xampp\htdocs\j4\libraries\src\Autoload\ClassLoader.php:56
D:\apachefriends\xampp\htdocs\j4\libraries\src\Autoload\ClassLoader.php:56
D:\apachefriends\xampp\htdocs\j4\tests\Unit\Libraries\Cms\Toolbar\ToolbarTest.php:238

As you can see, there is no reference to the file that is linked in the github issue that you linked anywhere. When I look at the line of the given file, the only code I get there is 

$toolbarFactoryMock->expects($this->once())

This happens because PhpUnit helpfully cuts of the part of the stack trace that belongs to itself, so you only see the place where it originates in your code -- usually an assertion failure. Of course, that does not work out will if there is an issue in PhpUnit itself. See https://github.com/sebastianbergmann/phpunit/blob/899db927169682058ed8875c3d0c7f30c1fa4ed2/src/Util/Filter.php for the relevant code. You might want to report an issue there to make the backtrace filtering less aggressive.